Author | Pool Han
Marvin probe image Source: NASA Goddard Space Center
From February 11 to April 5 this year, after nearly two months, NASA's Mars Atmospheric and Volatile Evolution Probe (referred to as Maven or "Marvin") successfully achieved "atmospheric braking", the lowest point of the orbit was reduced from 151 kilometers to 132 kilometers during the "atmospheric braking", reducing its orbital peak from 6050 kilometers to about 4570 kilometers, and the orbital period was shortened from the original 4.4 hours to about 3.7 hours.
The so-called "atmospheric braking", that is, the use of a small amount of friction in the upper atmosphere of Mars to change the satellite's orbit, is to allow Marvin to act as a communication relay between the lander and the rover during the 2020 NASA mission to Mars.
During this period, Marvin also obtained nearly two months of low altitude atmospheric data, providing a valuable research database for this altitude interval.
Fully functional Martian Atmospheric Laboratory
Dong Yaxue, an associate researcher at the Atmospheric and Space Physics Laboratory at the University of Colorado-Boulder, told China Science Daily that the goal of the Marvin project is very clear: to measure the upper atmosphere of Mars and its interaction with the solar wind, so as to study the long-term atmospheric evolution of Mars.
All instruments are designed closely around this goal, complete but very streamlined.
Marvin, for example, did not carry the instruments to take visible light images because it was not directly related to the evolution of the Martian atmosphere.
Moreover, other Mars projects have also taken a large number of images of Mars. Streamlined instruments avoid unnecessary failures, reduce interactions and limitations between instruments, and also save space for transferring data to bring higher precision back to Earth.
In addition, Marvin drew on and learned from several Mars exploration programs in NASA, ESA and the Soviet Union.
For example, ESA's Mars Exploration Satellite Mars Express does not have a magnetometer that can measure the magnetic field, which limits the study of mars' inductive magnetosphere and ion escape.
In order to study the escape of the Martian atmosphere, Xu Shaosui, an associate researcher at the Space Science Laboratory at the University of California, Berkeley, told China Science News that Marvin carried more complete particle and magnetic field measuring instruments than previous Martian atmospheric satellites, including the solar wind electron analyzer (swea), solar wind ion analyzer (swia), superthermal and thermal ion composition instrument (static), solar high-energy particle instrument (sep), magnetometer (mag), Neutral gas and ion mass spectrometers (ngims), Longmuir probe and wave analyzers (LPW), ultraviolet monitors (EUVM), and imaging ultraviolet spectrometers (iUVS).
These instruments can measure the distribution of electrons and ions in the solar wind and the Martian ionosphere, the composition of the Martian atmosphere, density, isotopes, electron temperatures, and more, as if a fully functional laboratory, providing primary information for building a martian atmospheric escape model.
Is Mars only an atmosphere away from man?
"Studying the Martian atmosphere is ultimately about studying mars habitability." Dong Chuanfei, an associate researcher at the Department of Astrophysics at Princeton University and the Princeton Plasma Laboratory of the U.S. Department of Energy, told China Science Daily.
Both Earth and Mars are in the solar system's habitable zone, which is defined as the existence of liquid water on the planet's surface.
Dong Chuanfei believes that understanding the escape and habitability of the Martian atmosphere can help us better understand the future evolution of the Earth's atmosphere. At the same time, it also provides a theoretical basis for human beings to modify Mars and make Mars suitable for human habitation.
Dong Yaxue said that there is a lot of geological evidence that early Mars had liquid water and a high-density atmosphere, and a dipole magnetic field like Earth, which was a humid and warm environment suitable for the evolution of life.
The current Martian atmosphere is thin, cold and dry, and only the relatively weak magnetic field is still present in some areas close to the planet's surface.
So, how is the water and atmosphere on the surface of Mars lost?
"From Marvin and other Martian rovers, we can extrapolate the escape rate of the Martian atmosphere, including neutral gases and ions, during the emergence phase." Dong Yaxue said that through observation data at different times, under different solar wind and solar radiation conditions, as well as the results of numerical simulations, scientists can quantitatively calculate the changes in Martian atmospheric escape with solar wind and solar radiation within a certain range, thereby extrapolating to the loss of the Martian atmosphere and changes in the atmosphere from the early to the present.
However, "there are still many limitations to our current understanding of the evolution of the Martian atmosphere."
Dong Yaxue believes that there is no direct measurement data for the conditions of the early sun and Martian atmosphere, and can only be speculated through theory and simulation, but different theoretical models may get different results, which is an uncertainty.
Does a dipole magnetic field like Earth's encourage or prevent the atmosphere from escaping? Is the absence of a dipole magnetic field causing a massive loss of the Martian atmosphere? None of these problems have been well addressed.
The solar wind strips the Martian atmosphere
Fruitful study of the Magnetic Field and Atmosphere of Mars
Xu Shaosui said that since entering orbit in September 2014, scientists have made a number of major discoveries about the Magnetic Field and Atmosphere of Mars with the help of Marvin observations, including the solar wind stripping away the early Martian atmosphere, thereby modifying the Martian climate.
Dong Yaxue et al. estimated the escape rate of atmospheric ions and the change of escape rate with solar radiation by observing and distinguishing various escape channels of ions.
Dong Chuanfei's recent numerical simulations have revealed the evolutionary relationship between the escape rate of the Martian atmosphere (neutral gases and ions).
He found that unlike today's Martian atmospheric escape, the escape of ions in the Martian atmosphere was much more important than the escape of hot oxygen in the early days of solar evolution.
Xu Shaosui told China Science Daily that Mars does not have a global dipole magnetic field like Earth, but has a regionalized crust magnetic field. These crust magnetic fields interact with the solar wind magnetic field to form an extremely complex and dynamic magnetic topology.
Therefore, magnetic topology research is an important part of the study of the interaction between Mars and the solar wind, and also provides important information for the analysis of the Aurora phenomenon of Mars, the ionosphere of the sun, and the mechanism of ion escape.
In recent years, Xu Shaosui and his collaborators have used solar wind electron analyzer and magnetometer data to establish a three-dimensional structure of magnetic topology near Mars, as well as changes in magnetic topology under extreme space weather conditions, and observed the morphological effects of the Martian crust magnetic field on the Martian ionosphere and inductive magnetosphere, such as the interaction between the crust magnetic field and the solar wind magnetic field can cause the twisting of the Martian magnetotail.
They developed a complete system to provide a more comprehensive database for future magnetic topology research and its applications.
In September 2017, Mars experienced the most intense interplanetary coronal mass ejection (icme, or "solar explosion event," the propagation of magnetic clouds from the sun's surface in interplanetary space) and solar flare events since the Marvin project began, with Multiple Marvin instruments making simultaneous observations of the Martian atmosphere and magnetosphere.
Based on Kepler's observations of Sun-like stars in the early stages of evolution, scientists infer that icmes were actually very frequent and common in the early stages of solar evolution (such as 4 billion years ago), and when ICME hit Mars, because Mars did not have the protection of the global magnetic field, ICME could interact directly with the upper atmosphere of Mars.
Compared with the general solar wind, the dynamic pressure of icme will be increased many times.
Dong Chuanfei said, for example, you just washed your hair and walked outside, the breeze blows slowly, and your hair is blown dry unconsciously.
However, if you hit a storm, maybe you will all be blown away, let alone just blow dry your hair.
The former is like the general solar wind, the latter can be understood as icme. So icme will increase the atmospheric escape rate of Mars many times, and the specific increase depends on the intensity of icme.
According to the Marvin study, published on the cover of the journal Science in 2015, even if the edge of an icme gently passes away from Mars, the atmospheric escape rate of Mars increases by more than 10 times.
It is conceivable that back 4 billion years ago, icmes were many times more frequent and intense than they are now, so ICME played a crucial role in the escape and long-term evolution of the Martian atmosphere.
It can be said that it is precisely because of the lack of protection from the appropriate strength magnetic field that the atmosphere of Mars was "blown" away by these violent early solar storms, which made Mars change from the warm and humid environment of the past to the cold and dry state it is today.
Recreate the Martian atmosphere
After understanding the effects of the solar wind and ICME on the Martian atmosphere, and the importance of a global dipole magnetic field to protect the Martian atmosphere and habitability, is it possible to artificially create a magnetic field to reshape the face of Mars?
NASA chief scientists James Green and Dong Chuanfei think it is possible.
They envisioned placing a powerful superconducting coil at the Lagrange point between Mars and the Sun, creating a giant artificial dipole magnetic field that placed Mars in its magnetic tail so that the Martian atmosphere would no longer be blown away by the solar wind.
If the Martian atmosphere escape rate becomes lower, and its surface is still supplying gas to the outside, the atmosphere will slowly accumulate, and at a certain point, the atmospheric temperature will rise.
As the temperature rises, the carbon dioxide ice on the north and south sides of Mars will slowly melt, so that greenhouse gases such as carbon dioxide in the atmosphere will increase, resulting in a continuous increase in atmospheric temperature, forming a positive feedback.
When the carbon dioxide melts, the ice at the north and south poles of Mars will begin to slowly melt into water or water vapor (also greenhouse gases), and this positive feedback will continue, and the surface of Mars will form rivers, lakes and even oceans of liquid water.
Green believes that this process will happen naturally, but it will take only 700 million years. Their plans enable this process to happen faster.
"The solar system is ours and we should go to them." He said.
Moreover, Dong Chuanfei said: "The atmospheric composition of the earth at the beginning was mainly carbon dioxide, which is very similar to the main components of the current atmosphere of Mars." So it's reasonable to speculate that when Mars has enough atmosphere and liquid oceans, at some point, a large oxidation event similar to that of Earth 2.6 billion years ago can occur on Mars, eventually producing a lot of oxygen. ”
Although Dong Chuanfei frankly said that with the current technical conditions, it is impossible to maintain such a strong magnetic field, he believes that with the development of human science and technology, this problem can eventually be broken.
It goes without saying that if that vision comes true and has a higher, warmer, and wetter atmosphere, the dream of a human move to Mars will become much easier to realize.
China Science Daily (2019-06-18 8th Edition Exploration and Discovery)
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